The present invention relates to an interference layer which acts as a coloring surface layer on aluminum items, said layer containing an aluminum oxide layer and, deposited on this, a partially transparent layer. The invention relates further to a process for manufacturing the interference layer according to the invention.
Interference layers which eliminate certain wavelengths of incident light by interference are known in optical applications as so called filters. Such filters are normally produced by depositing a high purity, thin metal layer on glass, subsequently depositing a dielectric layer and a further semi-transparent metal layer. The individual layers are normally deposited by PVD (physical vapor deposition) methods such as sputtering or vapour deposition.
The high purity, thin metal layer is normally of aluminum. The dielectric layers are normally layers of Al.sub.2 O.sub.3 or SiO.sub.2. Because of their small thickness, it is generally not possible to anodize PVD Al layers. Consequently, the dielectric layers are usually PDV-Al.sub.2 O.sub.3 or PDV-SiO.sub.2 layers. Depositing PDV-Al.sub.2 O.sub.3 or PDV-SiO.sub.2 layers is however expensive. Also, some dielectric layers deposited on aluminum surfaces by PVD methods do not adhere well. Metals such as high purity aluminum are normally employed for the semi-transparent layers.
A dielectric layer may be produced on an aluminum surface using known dc methods i.e. anodic oxidation of the aluminum surface using direct current and a sulphuric acid electrolyte. The resultant protective layer, however, exhibits a high degree of porosity due to the method employed. In order to produce surface layers with uniformity in color over large areas, it is necessary to achieve a constant thickness of interference layer over such areas. Using the dc method, however, it is difficult to produce dielectric layers of constant thickness over large areas.
The oxide layers produced in sulphuric acid are colorless and transparent only with high purity aluminum and AlMg or AlMgSi alloys based on high purity aluminum (Al.gtoreq.99.85 wt. %). With less pure materials, such as e.g. Al 99.85, Al 99.8 or Al 99.5, alloy constituents such as e.g. Fe or Si rich intermetallic phases may become incorporated in the oxide layer and lead to uncontrolled absorption and/or scattering of light and therefore to layers that are to a greater or lesser extent cloudy, or to layers with coloring which is uncontrollable.